Calibration Step Response
4.4 STATIC PRESSURE MEASUREMENT
It is also noted that homogeneous turbulence is only achieved at ten times the mesh length downstream, where the mesh length is the distance between the rod centres. Thus non- homogeneous turbulence exists at positions less than ten lengths downstream of the grid.
Snedden (1995) designed two sets of grids for two levels of turbulence intensity. The first set of rods were designed for a predicted 9.3% near homogeneous turbulence level, and used 2.4 mm diameter rods that were placed 10 mm apart and 6.21 mm perpendicular to the flow and 50 mm upstream of the test blade. The second set of rods was designed to generate non-homogeneous turbulence of 19.1%, and used 3.2 mm diameter rods placed 11.3 mm apart and 7 mm perpendicular to the flow and 25 mm upstream of the blade.
Turbulence measurements performed by Snedden (1998), however, showed that there was under-prediction of the turbulence levels. The grid that was designed to generate 9.3% was measured to generate 15.0%, and the grid designed to generate 19.1 % was measured to generate 25.5% turbulence intensity at the operating conditions needed. He also noted that the turbulence level measured without grids was 3.0%. This is summarised in Table 4-3.
Table 4-3: Turbulence Generating Rods
TURBULENCE ROD DISTANCE ROD PREDICTED MEASURED
GRID DIAMETER UPSTREAM SPACING Tu% Tu%
None N/A N/A N/A 0% 3.0%
Grid 1 2.4 mm 50 mm 10mm 9.3% 15.0%
Grid 2 3.2mm 25 mm 11.3 mm 19.1% 25.5%
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Figure 4-15: The SMR-95 test blade profile with static pressure tapping positions (reproduced from Snedden (1995))
This blade replaces the instrumented test blade for heat transfer when placed in the cascade. The cut-away in the cascade to make provision for the plunging process used in the heat transfer measurements (discussed in Section 4.2.8) is not needed for the static pressure tests, and the blade is fixed directly to a plate that covers the cut-away.
4.4.2SCANIVALVE
The data acquisition of the pressure measurements involves the use of the scanivalve and pressure transducers. The scanivalve is a device used to capture the data of multiple pressure readings, and is shown in Figure 4-16. It has two I-pole - 24-throw fluid switch wafers which are able to accommodate 24 readings per wafer taken in turn by a pressure transducer.
Since there are two wafers and two pressure transducers, the 42 pressure tappings can all be connected to the scanivalve using silicon tubes, and this allows simultaneous measurement and all the data can be captured in one test. This minimises the experimental error, as the period for capturing allows fairly consistent operating conditions for every static pressure tapping measurement. A rotor in the scanivalve steps between each reading, which COlll1ects a common port on the wafers to each of the 24 ports on the stators.
The tappings on the suction side were cOlll1ected to one wafer, while the tappings of the pressure side of the blade were cOlll1ected to the remaining wafer. A balance pressure that is equal to (or greater than) the average pressures measured is needed to act as a rotor thrust bearing on the scanivalve. The total inlet pressure that was measured by a Kiel probe was cOlll1ected and used to act as the balance pressure for the wafers. The two pressure transducers were cOlll1ected to the common ports on the wafers, such that when the scanivalve steps to a new position, the reading from the wafers are read by the transducers. Remaining open ports on the scanivalve were used to read the static pressures at the inlet and outlet of the cascade.
4.4.3ROSEMOUNT PRESSURE: TRANSDUCERS
The measurement of the static pressures along the blade profile is performed usmg two Rosemount Range 5 differential pressure transducers. The output voltage from the transducers is proportional to the differential pressure. The input voltage is supplied from the scanivalve, which is outputted as a current (4 - 20 mA). This current is passed through a resistor of 470
n,
and the resulting voltage is measured. The voltage range is between 1.88 V and 9.40 V.
The two ports on the transducer represents a high-pressure side (HP) and a low-pressure side (LP). The pressure difference between the two is measured. The common port from rotor on the scanivalve is connected to the LP side. A Kulite ITQ - 1000 total pressure transducer is used to measure the total cascade inlet pressure, and has a voltage range between 0 mV and 100 mV.
This is connected to the HP side of the Rosemount transducer, and the total inlet pressure is monitored. Once the data is measured and converted to a pressure reading, the static pressures are calculated by subtracting the total inlet pressure with the measured pressure.
The pressure transducers were previously set to a certain pressure range by De Villiers (2002), who noted that the static pressures never exceeded 40 kPa as measured by Stieger (1998). Thus a calibration screw on the transducer was used to set the zero point at 1.88 V (the lowest output possible from the transducer) and the maximum pressure value of 40 kPa was set to correspond with the maximum voltage output of 9.4 V.
4.4.4DATA ACQUISITION
The data acquisition system for the pressure measurement involves the use of an interface card, anAIDcard, and the data is read into the software program LabView.
Previously, the voltage output signals from the scanivalve were read into a PC 30 U card used for voltage protection from large voltage spikes before being fed into the PC 30 PGL AID card.
Cassie (2006) replaced the PC 30 U card with a more advanced PC 71 interface card, which not only served as voltage protection, but improved the ease of interfacing and noise reduction and ground noise pickup. The first two channels for the voltage inputs were used to read the outputs of the Rosemount pressure transducers, and the card was configured such that LabView could read the voltages.
The PC 30 card serves as the main board of operations for the data acquisition system, and is connected to the PC 71 interface card. The card has 16 single-ended inputs and allows for different output ranges, namely bipolar (range between -5 V and +5 V) and unipolar (range between 0 V and 10 V). There were a number of settings on the card that were configured for the pressure testing. By reviewing the PC 30 card manual supplied by Eagle Technologies, the jumper settings were con figured to allow for differential input, which reduces the noise that is usually experienced with the single-ended inputs. This reduced the 16 inputs down to eight analogue inputs. These were set for a unipolar input range to accommodate the voltage range of the Rosemount pressure transducers (range between 1.88 V and 9.4 V).
The software program LabView allowed for the controlling of the stepper motor used for the scanivalve, together with reading the voltages from the pressure transducers and monitoring the position of the scanivalve rotor. The program "Pressure Measurement", which was written by De Villiers (2002), also converted the voltage readings to pressure by using calibration constants acquired from calibrating the Rosemount differential pressure transducers and the Kulite pressure transducer.
Figure 4-17: Display interface for the program "Pressure Measurement"
As can be seen in Figure 4-17, the program allows for a number of tasks to be performed. By creating a program using built-in sub-programs called VIs (Virtual Instruments) and recreating the Pascal program written by Stieger (1998), De Villiers (2002) was able to provide and interface between the PC 30 card and LabView. The "Go Home" button ensures the scanivalve is in its original position to start any tests by stepping the rotor until that position is reached.
"Step Once" allows the user to step the scanivalve rotor once, which basically validates that all systems are working. "Perform Measurement" gives the command for the scanivalve to begin stepping between each port and recording the readings from the Rosemount pressure transducers. "Display Results" finally shows the results of the test and converts the voltage readings into pressure measurements.
The details of this program are given in Appendix 2 of De Villiers (2002).
CHAPTER
5
EXPERIMENTAL